Gallium nitride (GaN) thin film crystal draws attention as excellent blue light-emitting devices, has been used as a material for light-emitting diodes and expected as a blue-violet semiconductor laser device for an optical pickup. Recently, it draws attention as a semiconductor layer constituting electronic devices, such as high-speed IC chips, used for mobile phones or the like.
It is reported a method of obtaining a template substrate by depositing a seed crystal layer, such as GaN or AlN, on a single crystal substrate such as sapphire and of growing gallium nitride single crystal on the template substrate.
In the case that, however, the gallium nitride (GaN) seed crystal layer is grown on the substrate by vapor phase process by MOCVD and the gallium nitride single crystal is grown thereon by flux method, cracks are generated in the thus grown single crystal thick layer due to the difference of thermal expansion. For preventing the cracks, it is thus drawn attention the technique of reducing stress applied on the single crystal and of preventing the cracks, by spontaneously peeling the thus grown single crystal from the substrate.
According to Japanese Patent No. 4,422,473B, spaces are formed inside of a nitride layer of a nitride template substrate, GaN crystal is grown thereon by Na flux method, and the grown parts by flux method in the vicinity of the spaces are then separated (peeled) from the template. According to this process, however, complicated process steps are required, resulting in low productivity. Further, it is required to terminate the process of forming recesses in the nitride layer before the completion and the process control is difficult. Deviations are thus provided in the shapes of recesses so that the reproducibility in the peeling of the gallium nitride single crystal is low. Moreover, the film is grown over the whole surface of the MOCVD-GaN thin film so that the dislocation is not so reduced.
Further, according to another embodiment of Japanese Patent No. 4, 422, 473B, it is used, as a seed substrate, a nitride template including recesses in which sapphire is exposed and protrusions made of the nitride on the surface. GaN crystal is grown thereon by Na flux method and the portion grown by flux method in the vicinity of spaces formed by the recesses are separated (peeled) from the template. Since the sapphire-exposed part is present, however, the reproducibility is low when the gallium nitride single crystal is grown by liquid phase epitaxy by the flux method.
According to Japanese Patent Publication No. 2005-012171A, it is used, as a seed substrate, a nitride template substrate with a mask formed on the surface, and GaN crystal is grown by Na flux method to generate regions whose dislocation densities are low. According to the method, however, the reproducibility of the peeling of the grown gallium nitride single crystal is low and cracks tend to be generated if successfully peeled. Further, the single crystal includes periodically generated regions where dislocations are concentrated, so that it cannot be used over the whole surface.
According to another embodiment of Japanese Patent Publication No. 2005-012171A, a mask is formed on a surface of a sapphire body and a nitride layer is formed in its window parts to provide a seed substrate, which is used to grow GaN crystal by Na flux method to form regions with a low dislocation density. According to this method, however, the reproducibility of the peeling of the thus grown gallium nitride single crystal is low and cracks tends to be generated if peeled. Further, the single crystal includes periodically generated regions where dislocations are concentrated, so that it cannot be used over the whole surface.
Further, according to another embodiment of Japanese Patent Publication No. 2005-012171A, it is used a seed substrate including a sapphire body including recesses in which the sapphire body is exposed and protrusions made of a nitride. GaN crystal is grown thereon by Na flux method to generate regions whose dislocation densities are low. According to this method, the sapphire-exposed regions are present so that the reproducibility of the liquid phase growth of the gallium nitride single crystal is low.
Besides, Japanese Patent No. 4, 493, 427B describes a method of producing a layer of AlN single crystal.
It was further proposed a method of forming a low temperature buffer layer on a sapphire body, of forming a seed crystal layer thereon and of growing gallium nitride single crystal on the seed crystal layer. According to this method, the single crystal is peeled off using the stress applied by the cooling after the growth of the single crystal as a trigger (Japanese Patent Publication No. 2009-184,847A).